Fluorescein is a molecule reported in 1871, and has been widely used as a pH indicator or a labeling dye because of the high water solubility and high fluorescence quantum yield thereof. Since a calcium probe containing fluorescein as a mother nucleus was developed, there have been provided a large number of highly sensitive fluorescent off/on type probes utilizing intramolecular photoinduced electron transfer (PET), decyclization or cyclization of spiro ring, and the like. However, plural dyes containing fluorescein as a parent compound cannot be simultaneously used in molecular imaging, since fluorescence wavelengths thereof overlap with each other. Moreover, the probes utilizing the intramolecular photoinduced electron transfer suffer from a problem that such probes require precise design of the oxidation potential of the benzene ring, and therefore modification of the chemical structure is strictly limited.
A compound corresponding to the basic structure of rhodamine, pyronin Y (PY), of which oxygen atom is replaced with silicon atom (TMDHS) and application of this compound as a fluorescent probe have already been reported (Best, Q et al., Pacifichem 2010, subject number 2335, Dec. 19, 2010; Yuichiro KOIDE et al., Fourth Convention of The Japanese Society for Molecular Imaging, subject number P8-9, May 14, 2009). Further, there have recently been reported compounds usable as a mother nucleus of a red fluorescent probe, which correspond to fluorescein of which 10-oxygen atom of the xanthene ring is structurally modified (Chemical Communications, 47, pp. 4162-4164, 2011).